1. Field of the Invention
The present invention relates to a method and apparatus for forming a deposited film on a substrate by plasma chemical vapor deposition (hereinafter referred to simply as "plasma CVD"). More particularly, the present invention relates to a method and apparatus for forming a functional deposited film such as a semiconductor film on a substrate by plasma CVD, said deposited film being capable of use in the production of semiconductor devices, electrophotographic light receiving members (or electrophotographic photosensitive devices), image input line sensors, image pickup devices, photovoltaic devices (including solar cells), and the like.
2. Related Background Art
In the image-forming industrial field, for the photoconductive material to constitute a light receiving layer in an electrophotographic light receiving member, it is required to be highly sensitive, and have a high S/N ratio [photo-current (Ip)/dark current (Id)], absorption spectrum characteristics suited for an electromagnetic wave to be irradiated, quick photoresponsibility, and a desired dark resistance. It is also required not to be harmful to living things, especially the human body, upon use.
Particularly, for the electrophotographic light receiving member (hereinafter occasionally referred to as "light receiving member" for simplification purpose) used in an electrophotographic apparatus which is used as a business machine at the office, not causing pollution is highly important.
As an electrophotographic light receiving member, there can be mentioned electrophotographic light receiving members comprising a non-single crystalline silicon material such as an amorphous silicon (a-Si) material (the electrophotographic light receiving member comprising an amorphous silicon (a-Si) material will be hereinafter referred to as a-Si series electrophotographic light receiving member). For example, U.S. Pat. No. 4,265,991 discloses electrophotographic light receiving members having a photoconductive layer constituted by an a-Si material and which excels in moisture resistance, durability and electric and photoconductive characteristics.
A U.S. Pat. No. 4,788,120 discloses electrophotographic light receiving members having a surface layer constituted by an amorphous material containing silicon atoms (Si), carbon atoms (C), and hydrogen atoms (H) in an amount of 41 to 70 atomic %.
Based on these techniques, there have been realized desirable a-Si series electrophotographic light receiving members comprising an a-Si material, which are satisfactory in electric, optical and photoconductive characteristics, use-environmental characteristics and durability, and which provide a high quality reproduced image.
To effectively produce an electrophotographic light receiving member, an advanced technique is required. Particularly, in the case of an a-Si series electrophotographic light receiving member, it is necessary to have a greater area and thickness in comparison with other devices. As a result an a-Si deposited film formed as the light receiving layer must be uniform. It is also important to prevent abnormal film growth due to formation of a nucleus comprising foreign matter during the formation of the a-Si deposited film.
In view of this, there are various proposals for stably and efficiently producing a high quality a-Si series electrophotographic light receiving member on an industrial scale.
In the production of an a-Si series electrophotographic light receiving member, spherical growth defects occur during production. These cause a defective image comprising a so-called "white dot" (comprising a minute white dot) on a reproduced image. Such spherical growth defects are considered to occur due to abnormal film growth based on a foreign particle, deposited on a substrate on which the deposited film is formed, when a film deposited on the inner face of a deposition chamber is dislodged and lands on the substrate during the formation of the deposited film.
In order to prevent the occurrence of this problem, Japanese Unexamined Patent Publication No. 183871/1992 proposes a deposited film-forming method using a microwave plasma CVD apparatus having a microwave introduction means with two different regions. This document describes that by making the microwave introduction means have a face, which is contacted with plasma generated upon film formation, that has a value of 2.times.10.sup.-2 or less in terms of a product of dielectric constant (.epsilon.) and dissipation factor (tan .delta.) with respect to the frequency of a microwave used, stable discharge can be caused while preventing a film from depositing on the inner face of a deposition chamber. Thus, a deposited film having improved uniformity and which does not result in detective reproduced images reproduced can be formed. A technique of coating a microwave introduction means with an alumina ceramic by way of a plasma spraying process is described as an optiomal method. According to the technique described in this document, it is understood that a high quality deposited film with few spherical growth defects can be formed.
In recent years, there has been an increased demand for an electrophotographic apparatus with improved performance to provide a high quality reproduced image at a high speed with good durability. In addition, for the electrophotographic apparatus, there is an increased demand for decreased maintenance by improving the reliability of its constituents.
The electrophotographic light receiving member used in the electrophotographic apparatus, has been improved so that it can be continuously used under various environmental conditions over a long period of time without conducting maintenance works.
However, for the electrophotographic light receiving member thus improved, there are still some areas requiring further improvement.
Electrophotographic light receiving members produced by a conventional film-forming apparatus provided with a high frequency power introduction means, sometimes have a defective images having minute white dots depending upon the image-forming conditions employed.
From an industrial standpoint, when the productivity of an electrophotographic light receiving member produced is taken into consideration, to forming a desirable deposited film at a high speed (or a high deposition rate) is an important technical subject.
In order to form a deposited film at a high speed using the conventional film-forming apparatus provided with the high frequency power introduction means, increasing the amount of a film-forming raw material gas introduced into the deposition chamber (the reaction chamber) while increasing the quantity of a discharging power introduced into the deposition chamber is usually employed. However, in the conventional film-forming apparatus not only the electrode, but also the insulating material which covers the electrode are heated to high temperature as the quantity of the high frequency applied to the electrode is increased. As a result, problems related to the film-forming apparatus such as peeling of the film deposited on the insulating material, breakage of the insulating material due to thermal stress, and the like are liable to occur. In this case, it is difficult to achieve uniform film production at a high rate.